1. Field of the Invention
The invention relates generally to the field of marine seismic surveying. More specifically, the invention relates to methods and systems for transferring seismic data from a fixed-position data recorder or telemetry unit to a seismic vessel or other moving data collection device.
2. Background Art
Marine seismic data acquisition systems known in the art include recording buoys which are anchored to the sea bed. A typical recording buoy includes data storage equipment known in the art for storing seismic data. Such data storage equipment may include storage devices such as tape drives, magnetic hard drives, solid state random access memory and the like. The seismic data are acquired from various seismic sensors. In seismic data acquisition systems that use recording buoys, the seismic sensors are typically disposed in one or more cables positioned on the water bottom, such a cable being known as an “ocean bottom cable”. The sensors in the cable generate electrical and/or optical signals corresponding to the particular parameter being measured, the parameter being pressure, time gradient of pressure and/or a particle motion related parameter such as velocity or acceleration. Electrical and/or optical conductors in the cable transfer the signals generated by the seismic sensors to the data storage equipment in the recording buoy.
The data storage equipment on the recording buoy may be interrogated by a seismic data processing and recording system disposed on a seismic vessel or elsewhere. The interrogation may be performed by connecting a data transfer cable between the seismic data processing and recording system on the seismic vessel and the storage equipment in the recording buoy, or, preferably, the interrogation may be performed by wireless telemetry.
In a typical seismic survey using ocean bottom cables and recording buoys, a plurality of ocean bottom cables are deployed along the water bottom in a selected pattern, and the seismic vessel moves along the water surface in a predetermined pattern near the positions of the ocean bottom cables. The seismic vessel, or another vessel, tows one or more seismic energy sources. The seismic energy sources are actuated at selected times, and the signals generated by the sensors in the ocean bottom cables in response to detecting seismic energy are transferred to the storage equipment in the recording buoy. In survey techniques known in the art, the data stored in the recording buoys are accessed by interrogating the data storage equipment after completing the acquisition, or between parts of the acquisition. In any event, to establish wireless telemetry, the seismic vessel is moved to a location where wireless communication can be established between the recording buoy and the seismic vessel, and the vessel remains substantially at that location during the storage device interrogation.
It is desirable to be able to transfer seismic data from the recording buoy to the seismic vessel while the seismic vessel is moving during a survey using ocean bottom cables. The ability to transfer data while the seismic vessel is moving would enable, among other things, more rapid quality evaluation of the seismic data. Quality control of the data during acquisition could provide, for example, that the seismic vessel is able to return immediately to any portion of the predetermined pattern to reacquire the seismic data in the event any of the data in such portion are substandard. Such ability may provide cost savings by reducing the operating time for the seismic vessel.
Wireless telemetry devices known in the art for transferring data between two substantially fixed position devices are disclosed, for example, in U.S. Pat. No. 4,663,744 issued to Russell et al. The Russell et al. '744 patent discloses a real time seismic telemetry system including a central command station for communication with a plurality of remote data acquisition units, such as recording buoys. The central command station has a command unit for controlling the operation of a transmitter, for providing instructions to the data acquisition units. The data acquisition units receive the instructions on a receiver and process the instructions in a logic control circuit. Seismic data are detected by one or more sensors and converted to digital data for transmission through a transmitter which is tuned to a discrete channel for each data acquisition unit. The command station has a PCM receiver tuned to each of the channels for demodulating the data stream therefrom. A digital receiver is provided in the command unit for synchronizing and processing the data. The digital receiver synchronizes both to the bit rate and to the beginning and ending of the digital word such that data contained in the digital word can be multiplexed onto a data bus. The data bus is controlled by an external storage/control for storage of the data from all of the digital receivers for all of the discrete channels.
Direct adaptation of such wireless seismic telemetry systems known in the art to transfer of seismic data (or between two moving vessels) has proven difficult because seismic telemetry systems known in the art for transmitting data between fixed locations typically use directionally sensitive antennas. Directionally sensitive antennas have a large magnetic dipole moment along essentially one direction and provide relatively high signal gain along that direction, but provide substantially no signal sensitivity along any other direction. Thus, it is impracticable to use fixed position, directionally sensitive antennas to communicate signals between two devices that move relative to each other. Omni directional antennas provide substantially uniform signal gain in any direction from the antenna, but the gain is relatively small, and for high data-rate telemetry, such as would be used in multi-channel seismic data acquisition, low signal gain would require relatively high telemetry transmitter power. Because typical recording buoys are powered by batteries, it is desirable to keep the power consumption of the telemetry system as small as practical. Therefore, the power output of the telemetry transmitter in a typical recording buoy would be limited. Accordingly, there is a need for a seismic data telemetry system that enables signal communication between a recording buoy and a moving seismic vessel that does not require a high power transmitter.